In gas turbine engines, disks which support turbine blades rotate at high speeds in an elevated-temperature environment. These turbine disks encounter different operating conditions radially from the center or hub portion to the exterior or rim portion. The turbine blades are exposed to high temperature combustion gases which rotate the turbine. The turbine blades transfer heat to the exterior portion of the disk. As a result, these temperatures are higher than those in the hub or bore portion. The stress conditions also vary radially across the disk.
Until recently, it has been possible to design single alloy disks capable of satisfying the varying stress and temperature conditions across the disk. However, the need in modern gas turbines for increased engine efficiency and improved engine performance now dictates higher engine operating temperatures. As a result, the turbine disks in these advanced engines are exposed to higher temperatures than in previous engines, placing greater demands upon the alloys used in disk applications. The temperatures at the exterior or rim portion may reach 1500.degree. F., while the temperatures at the inner or hub portion will typically be lower, e.g., of the order of 1000.degree. F.
In addition to this radial temperature gradient, there is also a stress gradient, with higher stresses occurring in the lower temperature hub region, while lower stresses occur in the higher temperature rim region in a typical disk. These differences in operating conditions radially across a disk result in different mechanical property requirements in the different disk regions, with the rim portion subjected to severe creep and hold time fatigue crack growth conditions and the hub portion subjected to severe fatigue and high stress conditions. In order to achieve the maximum operating conditions in terms of efficiency and performance in an advanced turbine engine, it is desirable to utilize disk alloys having excellent hold time fatigue crack growth resistance and high temperature creep resistance in the rim portion while having high tensile strength and fatigue crack-resistance at moderate temperatures in the hub portion.
Various solutions have been attempted to achieve a disk capable of meeting the demanding mechanical properties requirements encountered by a turbine disk in an advanced turbine engine at temperatures up to about 1500.degree. F. One solution for meeting these higher operating temperatures required in these more efficient and more powerful advanced engines is to increase the weight of the disk made from an alloy having sufficient high temperature stability in order to reduce stress levels. This solution is unsatisfactory for aircraft due to the undesirable increase in the weight of the system which negates advantages of increased power and efficiency.
Another approach has been to make a single alloy disk whose different parts have different properties. U.S. Pat. No. 4,608,094 which issued Aug. 26, 1986, outlines a method for producing such a turbine disk. The disk is made from a single alloy which has been worked differently in different regions to yield different mechanical properties. However, such a disk is necessarily subject to the limitations of the single alloy employed. Alternatively, a single alloy disk may have different portions subjected to heat treatment at different temperatures, or at the same temperatures for different times as described in U.S. Pat. No. 4,820,358. Such a differential heat treatment will produce a disk having different mechanical properties in different portions. However, the disk is still subject to the previously mentioned limitations of the single alloy used.
U.S. Pat. No. 3,940,268 which issued Feb. 24, 1976, provides for turbine disk/blade assemblies. It discloses a disk of powdered metal material connected to a plurality of radially, outwardly-directed airfoil components located in a mold and metallurgically bonded during hot isostatic pressing ("HIP") formation of the disk element. While blades can be bonded to a disk of a different material by the method set forth in the '268 patent, hybrid or composite turbine rotor structures formed by such methods may lack precision and dimensional control between adjacent airfoil components. Such control is required to maintain the desired gas flow through adjacent passages of the airfoil components connected to the disk. The '268 patent does not, however, provide a means for joining separate portions of a disk.
Another approach is to use a dual alloy disk wherein different alloys are used in the different portions of the disk, depending upon the properties desired. The disk has a joint region in which the different alloys are joined together to form an integral article. Various methods for fabricating dual alloy disks have been suggested or evaluated. The heretofore previously known fabrication techniques for dual alloy disks have all been limited because of special problems related to configuration, cost or alloy composition. As employed herein, the term joint refers to a metallurgical joint wherein the joined members are held together by fusion of their metals or a third metal, as in the case of a diffusion braze or diffusion weld, as opposed to a mechanical joint wherein the joined members are held -in contact by mechanical means such as bolts or rivets. The joint and region of altered metal adjacent thereto are referred to as the joint region.
The concept of forming a rim portion of a disk with a coarse grain and a central portion of a disk with a fine grain is disclosed in NASA Report No. CR-165224 entitled "Development of Materials and Process Technology for Dual Alloy Disks". The report indicates that the rim portion of a disk is formed from powdered metal by HIP of Powdered metal. The hub portion of the disk is then filled with metal powder and is enclosed in a container. The enclosed rim portion and the powdered metal are then subjected to a HIP operation to produce a dual alloy turbine disk. The disadvantage of HIP is that any impurities present at the joint prior to HIP will remain there. In a process analogous to HIP, two wrought sections are joined together by a HIP operation. This technique requires a gas-tight enclosure, such as a separate can, or a weld or a braze, around the exposed sides of the joint regions. In yet another variation of the HIP method, an annular ring of powder is placed between two wrought members and the assembly is subjected to HIP.
Fusion welding also has been suggested, but the nickel-base superalloys of the type used in disks are difficult to weld by this method.
Inertia welding is a Possible alternative. However, with very dissimilar alloys, there is a potential for uneven flow, inadequate joint clean-up and incipient melting in the heat-affected zone.
Another technique for bonding parts made of different alloys is by diffusion bonding as applied to nickel-base alloys. However, this method is currently not considered sufficiently reliable for producing dual alloy disks.
Another method is referred to as bicasting, or casting one portion of an article, such as a rim, directly against another portion, such as a wrought or a forged hub. This method provides an undesirable mechanical joint, as distinguished from a metallurgical joint. Further, the fact that one portion of the article is necessarily cast means that at least that portion may contain characteristic casting defects, such as inhomogeneities, shrinkage, inclusions and porosity. The presence of such defects is undesirable for a high speed rotating part.
Billets made by coextrusion and isoforging, in which a core is made from one alloy and an outer portion is made from another alloy, have been manufactured with relatively little difficulty. However, additional development is needed to develop forging procedures to control the precise location and shape of the interface between the joined parts.
Explosive welding has been used to weld combinations of dissimilar alloys. This process has been found to be useful for cladding one alloy onto the surface of another. Such a process is, however, not presently usable for joining dual alloy disks, as the configuration of the joint region of such disks is not suitable for the introduction of explosive energy for bonding a hub to a rim.